Much of the radio transmitting and receiving equipments in operation today use broadband circuitry which reduces the number of components, particularly the more expensive ones, and is simpler, lighter, smaller, less expensive and particularly attractive for use on aircraft. However, by its very nature a broadband transmitter generates unwanted in-band radiation from its power amplifiers. This does not generally create a serious problem except when the transmitter is located near a co-located receiver. Since many modern aircraft now carry a number of broadband transceivers which often operate in close proximity at the same time, the unwanted in-band radiation from the power amplifiers of one transmitter may interfere with nearby receivers operating in the same frequency band.
The present invention provides a cure for presently installed broadband equipment, and it is designed for connection to the existing equipment without the need for modifying any internal circuit. The present invention provides a dynamic filter system which is connected as a unit between the transmitter power amplifier and antenna for substantially reducing transmitted noise and spurs. The invention attacks the noise problem at its source, by dissipating the original carrier and regenerating a noise free carrier modulated with the original signal. The system has the capability of providing gain.
The primary noise problem with existing radios results because R.F. power is generated at fairly low power levels, usually in the low milliwatt region. Broadband amplifiers are then used to boost the power to high levels, e.g., 100W, and gains of 40-60 db between the R.F. oscillator level and the R.F. output level are not uncommon. Such high R.F. gain, since it acts on noise as well as signal, is directly responsible for the amplification and eventual radiation of broadband noise. This invention provides a unique means for materially reducing this noise without a power loss.